The invention relates to an apparatus and method for processing a magnetic tape housed within a cassette, in particular for performing all normal functions such as record, playback, fast forward, rewind, search and cue etc. on a conventional audio cassette.
To overcome problems associated with prior art so-called "open reel" or "reel-to-reel" tape recorders, Philips Export BV of The Netherlands introduced the so-called "Compact Cassette". The term "Philips", "Compact Cassette", "Digital Compact Cassette" and "DCC" as used herein are trademarks or terms associated with Philips Export BV of The Netherlands. This cassette has been in use for nearly 30 years and was originally developed for low fidelity dictation purposes, where recording fidelity was not a priority. Since then, use of the cassette expanded considerably to include high fidelity music recordings and many problems and shortcomings associated with these new uses were recognized and overcome, but some still remain.
In particular, problems associated with accurate azimuth alignment of the tape as it passes across a magnetic head have not yet been solved satisfactorily at a reasonable cost. Tape alignment is controlled primarily by at least one fixed tape guide located "upstream" of the magnetic head, and by location of the cassette body or shell within the cassette compartment. This is because tape guidance is partially dependent upon guide structures which are integral to the cassette shell itself, and thus errors in the location of the cassette shell and manufacturing errors in the shell itself can contribute to azimuth errors as the tape passes across the magnetic head. Because the tape has a relatively slow tape speed and narrow audio track width, these alignment errors can aggravate azimuth errors in the tape which can contribute to a loss in recording fidelity, particularly for relatively high frequencies. This problem was recognized and many attempts have been made to reduce the problems of azimuth misalignment in tape transports. One approach was to improve accuracy of manufacturing of the cassette shell by using physically stable materials manufactured to close tolerances. While this improved azimuth alignment accuracy to some extent, accumulation of mechanical positioning tolerances between the cassette shell and the cassette compartment, and in other areas tended to detract from recording/playback fidelity. In some expensive recording and playback apparatus, the manufacturer provides a user adjustable head alignment system which, while permitting adjustment for a particular cassette, would require re-adjustment for another cassette or an opposite side of the same cassette due to small dimensional variations among individual cassette shells which can affect effective tape-to-head alignment, particularly when a tape is played back on a machine different from the one it was recorded on. More complex automatic self-aligning head mechanisms require complex electronic and servo-devices which increase the costs considerably.
The above description relates primarily to analog audio cassette tapes, and while digitally recorded tapes overcome many of the problems inherent with analog tapes, initial accurate alignment of a digital tape with the heads would considerably reduce read/write errors of digital data and thus augment error correction systems in the digital audio processor to provide a robust high fidelity recording and playback system.
One approach to avoid cassette shell problems is to extract the tape from the cassette for recording and playback. In the video cassette field, it is well known to provide a cassette which is specifically adapted to permit the tape to be automatically extracted or withdrawn from the cassette and loaded onto a drive mechanism for processing. Such cassettes are specially adapted to permit easy withdrawal of the tape by providing a hinged or sliding gate mechanism along a front edge of the cassette, which is opened to facilitate access for extraction of the tape when appropriate. The tape is extracted from the cassette because the video head is too large to be accepted in the cassette and requires an extensive tape wrap and precise head to tape alignment for proper recording and playback of video and audio signals. There are many patents on video cassette mechanisms, all of which show moveable gates to permit easy access to the tape for extraction. Some advanced audio cassette also show movable gates to permit easy access to the tape.
U.S. Pat. Nos. 3,797,036 (Eibensteiner) and 4,323,936 (Beitler et al) both issued to U.S. Philips Corporation, and U.S. Pat. Nos. 3,825,944 (Terao et al) and 4,413,293 (Hathaway) issued to Victor Company of Japan and Ampex Corporation respectively, disclose video cassette transport mechanisms. U.S. Pat. Nos. 4,050,087 (Kishi et al) and 4,130,848 (Amans et al) both issued to Sony Corporation disclose an audio transport and cassette termed Elcaset (Trademark). U.S. Pat. No. 4,814,910 (Kaku et al) issued to Hitachi Corporation discloses a Betamax (Trademark) video transport, and U.S. Pat. No. 4,933,788 (Patel et al) issued to Seagate Corporation, discloses an audio/data transport, the so-called DAT (Digital Audio Tape) cassette.
The Compact Cassette was not designed for easy, automatic tape extraction and has a fixed processing edge portion with access openings or "wells" to permit access to the tape for engagement by one or more magnetic heads, and at least one set of a pinch roller and capstan for driving the tape. The wells at the processing edge portion of the cassette are defined by webs which extend transversely between edges of the generally flat, broad parallel faces of the cassette. These webs are intended to enclose part of the exposed portion of tape running across the processing edge of the cassette body to minimize tape spillage and are not normally in contact with the tape surface. Thus, such webs generally have relatively rough surface features, often including a seam defining the shell halves, which in applicant's opinion, present a potential hazard of tape damage to a length or loop of tape as it is extracted from the cassette because they are not intended to guide the tape.
The present applicant is aware of two patents showing structure for extracting the magnetic tape generally perpendicularly from a conventional Compact Cassette, and for loading the tape into a transport mechanism for processing the tape externally of the cassette. Neither of the patents disclose that the tape is extracted for improvement of tape-to-head alignment accuracy or recording/playback fidelity. The first of these patents, namely U.S. Pat. No. 3,612,539 is issued to Blaupunkt Werke GmbH, and the inventor is Peter Bragas. This patent provides a long, variable length of extracted tape loop for educational use, in which a time-delayed effect between different sound tracks on the tape can be achieved by scanning the extracted tape. The tape is drawn from the cassette through an end well of the cassette and along a path generally at right angles to the cassette. It would appear that the tape is drawn across inwardly facing edges of a web which could result in damage to the tape surface. Few details are given on the means of locating the magnetic heads adjacent the tape, or of driving the tape.
The second patent is U.S. Pat. No. 3,902,680 issued to the Bell and Howell Company, in which the inventor is Joseph J. Neff. This patent discloses a relatively complex mechanical "escalator" mechanism for drawing a tape through the center well of the cassette for the purpose of improving versatility and performance and overcoming problems with the size of the magnetic heads and speed stability inherent in the early cassette mechanisms. When the tape is extracted from the cassette, the escalator mechanism skews the tape vertically from an original plane, i.e. moves the tape laterally out of a plane containing edges of tape wound on the supply and take-up reels. Since tape or back tension is almost always necessary during a loading process to keep the tape taut while being threaded, chances of tape damage are increased if the tape is skewed vertically while back tension is applied. While this reference shows the extracted tape loop passing around rollers as the tape leaves and enters the cassette well, the rollers do not pass into the well, and thus it appears that the tape contacts inwardly facing front edges of a web adjacent the well, which contact could damage the tape due to the relatively rough surfaces of these edges which originally were not intended to contact the tape surface.
In both patented structures described above, the tape drawn from the cassette passes in a generally U-shaped tape loop and the only areas available for magnetic head contact with the tape are the "straight" sides of the loop. In this arrangement, relatively high tape back tension on the supply reel is necessary to provide sufficient tension for proper head contact which can aggravate wear of the tape. To provide clearance for the escalator mechanism to withdraw the tape, the magnetic head assembly is positioned initially remotely from the tape, and then is moved to contact the tape when recording/playback operations are required. Furthermore, in both structures, the tape extraction mechanism extends some considerable distance from the cassette which would result in a bulky mechanism which would clearly be unacceptable for many applications of portable devices commonly used nowadays.
Magnetic tape transports which use a conventional capstan and pinch roller drive for transportation of the tape can use one of two common methods to generate contact pressure between the tape and a magnetic head to ensure proper tape-to-head contact for scanning of the tape. The first method is employed on high performance tape transports, and involves maintaining a specific tape tension across the heads so that the tape is wrapped sufficiently tightly across all magnetic heads. This tape tension may be generated by a suitable amount of reverse torque or frictional drag on the supply reel hub on single capstan drive transports, thus creating back tension. 0n more sophisticated transports, dual capstan drive is used to generate tape tension between a supply side capstan and a take-up side capstan. The dual capstan drive introduces a drag on the tape by feeding the tape through the supply capstan and corresponding pinch roller assembly slightly slower than through the take-up capstan and corresponding pinch roller assembly.
However, on many tape transports such as those designed for the Compact Cassette and Digital Compact Cassette, the use of a back tension control mechanism or dual capstan drive method increases the cost of the transport itself somewhat and is therefore omitted on lower performance transports. To compensate for such mechanical deficiencies, the second method attains acceptable tape-to-head contact in such cassette apparatus by using a pressure pad assembly provided in the center head well of the cassette body. The pad assembly usually consists of a piece of low-shedding felt cut to specific dimensions, mounted on a resilient metallic spring plate which can engage the tape while the tape is passed across the magnetic head when inserted into the head well of the cassette. While the presence of the pressure pad assembly ensures that the cassette will function properly in a range of tape transports having vastly different qualities, on transports which have proper tape tension control as described above, the pressure pad assembly is unnecessary and in fact becomes a source of mechanical problems for accurate tape transport. Because the pressure pad assembly must be a loosely anchored structure, it can be a potential cause of tape skewing and misalignment. In addition, physical contact between the pad material and the moving tape surface introduces what is known as "scrape flutter" into the tape itself, i.e. as the tape scrapes by the pad, frictional vibration causes the tape portion in the vicinity of the pad to resonate or flutter, thus muddling the sound quality in an analog cassette apparatus. In a digital cassette apparatus, severe scrape flutter can cause read/write errors of digital data.
To eliminate adverse effects of the pressure pad on high performance equipment, an additional device known as a pressure pad lifter is used to lift the pad away from the tape when inserted magnetic head(s) engage the center head well of the cassette. Pad lifters are used only on apparatus which have some form of tape tension control to maintain tape-to-head contact in the absence of the pressure pad in the tape path. Although a number of variations of pad lifters have been used in the prior art, all forms of the pad lifter known to the applicant cooperate with the inserted magnetic head(s) and are usually mounted on the head(s) or closely adjacent the head. The pressure pad is lifted away from the tape, without interference with the tape, by engaging margin portions of the pressure pad on opposite sides of the pad material which extend beyond the width of the tape. Pad lifters have appeared in the forms of a shield, nodules and a forked guide. The pad lifter in prior art defeats the pressure pad when the inserted magnetic head(s) engage the tape.